Spark plugs of various constructions have been proposed for improving the operating efficiency of an internal combustion engine. Certain prior spark plugs have electrode arrangements for producing a plurality of sparks during each firing cycle of the plug as, for example, the construction shown in J. A. Stahr U.S. Pat. No. 2,208,059, issued July 16, 1940. Other spark plugs have electrodes constructed and arranged to be self-cleaning, as shown in T. T. Bathurst U.S. Pat. No. 1,384,818, issued July 19, 1921. My U.S. Pat. No. 3,872,338, issued Mar. 18, 1975 discloses a spark plug in which both of the electrodes are encased in a ceramic material to eliminate electric and chemical corosion, thereby prolonging the operating life of the plug. Still other prior spark plugs are provided with auxiliary members for modifying the propagation of the combustion flame, as shown in K. Yamazaki U.S. Pat. No. 3,965,384, issued June 22, 1976. While the prior spark plugs provide varying degrees of improved operating results, they do not result in a significant increase in the horsepower output of the engine under given engine operating conditions. A spark plug made in accordance with this invention is of simple construction, has a long operating life, and results in an unexpectedly large increase in engine horsepower output at the same fuel consumption rate as when the engine is equipped with conventional spark plugs.